The present invention relates to an arrangement in the wet end of a paper machine for transferring a web from the former to the press section, said former comprising a twin wire zone mutually defined by an upper wire loop and a lower wire loop, whereafter the web is arranged to follow the upper wire to a transfer point where the web is transferred from the underside of the upper wire onto the top side of the weave included in the press section.
The transfer of a web formed in the wire section of the paper machine to the press section is an important phase which greatly affects the reliability of the paper machine running. A reliable transfer of a wet and weak web from the former to the press section requires a good grip of the web onto the surface of the pick-up felt or the surface of the transfer belt. This important process phase usually includes also web edge trimming and running of the web into the pulper in break situations.
In a number of twin wire formers known in the art the web is at the end of the twin wire zone released from the upper wire and made to follow the lower wire, from the downwards slanting run whereof it is picked in a transfer point with the aid of a pick-up roll onto the underside of a pick-up felt, said felt acting also as a water receiving press weave in the first press nip. Alternatively, a web supported by the lower wire can be conducted, together with the transfer belt, into a pre-press nip, in which it is made to engage with adhesion onto the underside of the water non-receiving transfer belt with smooth surface. At the end of the twin wire zone, in the proximity of the release point a suction roll and/or suction boxes is/are in general placed inside the lower wire loop, the function of which is, not only to increase the dry matter content of the web, but also to secure that the web follows the lower wire after the release point. When modern suction rolls and suction boxes acting on high underpressure are used to intensify water removal, problems may arise, especially at high speeds of running, in the transfer point where the web is intended to be released from the lower wire and made to grip reliably onto the underside of the transfer weave.
In order to solve problems related to web transfer, arrangements are disclosed in U.S. Pat. Nos. 4,116,763 and 5,736,011, in which the web is after the twin wire zone arranged to follow the upper wire, from the underside of which it is transferred in the transfer point onto the top side of the transfer weave. At the end of the twin wire zone, the web is separated from the lower wire with the aid of a suction roll or a suction box arranged inside the upper wire loop. In said designs the problem still is how to achieve sufficient grip of the web onto the upper wire with the aid of conventional suction rolls and suction boxes, because of which the space from the end of the twin wire zone to the pick-up point has to be kept short and/or the web has to be supported by means of a suction box or suction roll positioned on the side of the upper wire. When the space is short, the trimming of the web, the removal of the trimmed strips and the running of the web into the couch pit are difficult to arrange.